1. Field of the Invention
This invention relates to a three-dimensional shape measurement system, and more particularly to a three-dimensional shape measurement system which produces three-dimensional information regarding a solid object under examination by receiving light reflected from the object, converting the received light into an electric signal, and processing the electric signal.
2. Description of the Prior Art
Numerous methods have been developed for optically measuring the three-dimensional shape of solid objects in a non-contact manner. Among the various applications of such optical three-dimensional shape measurement methods in the field of medicine, one that has drawn particular attention in recent years is a measurement apparatus for obtaining three-dimensional information regarding the fundus of the human eye.
Examination of the eye fundus provides important information not only for ophthalmologists but also in the field of internal medicine, where it is used in the diagnosis of hypertension, diabetes and other diseases. Photographing of the eye fundus with an eye fundus camera or the like has thus become a widely used medical procedure. Quantitative information regarding the pattern of irregularities (bumps and depressions) on the eye fundus is highly useful in the diagnosis of glaucoma and retinal detachment, as well as various types of edemas and tumors, and may be critical in preventing blindness. Because of this, attempts have been made to obtain three-dimensional image information regarding the eye fundus in addition to the ordinary two-dimensional image information.
One method of conducting such three-dimensional measurement involves projecting a specific grating pattern on the eye fundus, measuring the grating image shift by observation of the projected pattern from a direction differing from the direction of projection by a prescribed angle, and thus ascertaining the pattern of fundus depressions and the like. ( See U.S. Pat. No. 4,423,931.)
Another method used for three-dimensional measurement of the eye fundus is based on the principle of stereoscopic photography. For example, a fundus camera is used to take two photographs at different angles from different entrance pupils and the resulting photographs are image-analyzed to ascertain the amount of fundus irregularity. Moreover, there has recently been developed a system that enables three-dimensional information to be obtained automatically by linking a TV camera with a computer, thus eliminating the need for photographic film. (See U.S. Pat. No. 4,715,703.)
However, in all of these prior art methods the imaging optical system of the eye is used nonaxially. As a result, the spatial resolution, accuracy and reproducibility of the three-dimensional measurement is poor and the processing of the signals is complex. Thus none of the methods is entirely satisfactory in terms of practical utility.
On the other hand, apparatuses that use the laser scanning method for in vivo observation of the eye fundus are becoming increasingly popular. (See Japanese Patent Public Disclosure No. Sho 62-117524, corresponding to U.S. Pat. No. 4,764,005, Japanese Patent Public Disclosure No. Sho 64-58237, corresponding to U.S. Pat. No. 4,854,692.) An attempt has been made to use this type of apparatus for measuring three-dimensional shape through a process that involves taking a plurality (e.g. 32) tomographic images of the eye fundus while progressively shifting confocal apertures located in front of a light detector and then reconstructing the fundus image. (See SPIE Proceedings, Vol. 1161, Sessions 7 and 8.) Since this laser scanning method uses the imaging optical system of the eye in a coaxial manner, the measurement accuracy is correspondingly high.
As a practical matter, however, the taking of a plurality of images requires the use of a large-capacity memory device, while the fact that the time needed for the measurement is not negligible means that the measurement accuracy is apt to be impaired by eye movement.
In response to this situation, the applicant therefore earlier proposed a highly innovative system for measuring the three-dimensional shape of objects that is based on a totally new principle. (See Japanese Patent Public Disclosure NO. Hei 1-113605, corresponding to U.S. Pat. No. 4,900,144 and Optics Communications, Vol. 74, 1989, pp. 165-170.) The principle of this invention is illustrated in simplified form in FIG. 4.
In FIG. 4, the laser beam from a laser beam source 60 is two-dimensionally deflected (scanned) by X-Y scanning unit 61 and the scanned beam is projected onto the fundus of an eye 62 under examination. The light reflected and scattered by the fundus passes back through the X-Y scanning unit 61, is split by a half mirror HM, and focused in the vicinity of two confocal apertures A1 and A2.
The apertures A1, A2 are disposed on the optical axes so as to sandwich the fundus focal plane. As a result, the increase and decrease in light intensity caused by irregularities (bumps and depressions) on the object (the eye fundus) are in reverse relationship between the two apertures.
The intensities of the light passing through the apertures A1 and A2 are detected by detectors D1 and D2, which produce output signals carrying irregularity information. Since the intensity of the output signals is also completely dependent on the reflectance of the object, the signals are sent to a signal processing unit 63 in which division processing is electrically conducted therebetween so as to cancel out the effect of the object reflectance and obtain information relating solely to the object irregularities. The result of the measurement or a measurement image is displayed on a TV monitor 64.
FIG. 5 shows the intensity variation at the two detectors, in which the horizontal axis indicates the distance along the optical axis and (A1) and (A2) on the z axis represent the positions of the two apertures. Defining f1 (z) and f2 (z) as functions of the light intensity variation at the apertures, the signal intensities I1 and I2 at the detectors D1 and D2 can be expressed as follows EQU I1=f1(z) * Io(x, y) (1) EQU I2=f2(z) * Io(x, y) (2)
where Io(x, y) is the variation in intensity of the light from the fundus and is dependent on position (x, y).
Therefore, by carrying out the following calculation involving division, it is possible to cancel out the effect of the reflectance dependent on position (x, y) and to obtain information of a form relating to the height or depth z. EQU I1/I2=f1(z)/f2(z) (3) EQU or EQU (I1-I2)/(I1+I2)=Af1(z)-f2(z)U/Af1(z)+f2(z)U (4)
Since applying this method for measurement of the three-dimensional shape of the eye fundus enables the imaging optical system of the eye to be used coaxially, it is able to provide high measurement accuracy. Moreover, it does not require a large-capacity memory, can be realized by simple signal processing in the signal processing unit 63 and enables the measurement of the object to be conducted in such an extremely short time as to provide substantially real-time observation of the three-dimensional shape.
However, the method does have a drawback in that when it is actually applied in a three-dimensional shape measurement system the effective range of the three-dimensional measurement in the z direction is limited to the region between (A1) and (A2) in FIG. 5. It is thus difficult to conduct accurate measurement when the focal plane of the object being measured falls outside the effective measurement range.
Moreover, where the measurement is carried out with respect to an eye fundus and has to be conducted in vivo, the measurement is complicated by the fact that the variation in refraction among the eyes of different patients causes variation in the focal plane. While it has been the practice to internally adjust the optical system in the instrument as required for examining patients with myopia, hyperopia and other refractive problems, an inexperienced operator is apt to have difficulty making the complex adjustments required so that erroneous measurements are likely to occur.
Another problem relates to the division processing required for implementing the method. High-speed, broad-band division circuits generally have a narrow input dynamic range and, as a result, the processing accuracy decreases when the intensity level of the detection signal falls outside this range.
The intensity level of the detection signal just before it is input to the division circuit is greatly dependent on the reflectance of the patient's eye fundus, the eye refractive condition, the transparency of the eye optical system and other factors. While it has been possible to adjust the detection signal level by changing the multiplication factor of the detectors themselves or that of the following circuitry, this is a complicated operation that may be a cause for measurement error if not conducted properly.